The present invention generally relates to electrochemical cells. More specifically, the present invention relates to an improved electrolytic manganese dioxide (EMD) for an alkaline electrochemical cell.
Manufacturers of alkaline electrochemical cells are constantly attempting to increase the service life of the cells, and more particularly, the high-rate service life of their cells to meet the demands of current battery-operated devices, which draw increasingly larger current levels from the batteries. Because the outer dimensions of the battery are generally fixed by various standards, battery manufacturers cannot arbitrarily increase the outer dimensions of the battery in order to accommodate more of the electrochemically active materials in their batteries. Thus, substantial effort has been made to make more efficient use of the space provided in the interior of the battery so as to enable more electrochemically active materials to be contained inside of the battery. Such efforts have included minimizing the volume occupied by the current collector and seal that are contained inside of the battery as well as increasing the density of the electrochemically active materials at the expense of other component materials, such as electrolyte or conductive agents. Other efforts have focused on increasing the high-rate discharge efficiency by utilizing electrode constructions that optimize the interfacial surface area between the positive and negative electrodes. In addition, battery manufacturers have studied the electroactive materials themselves to increase their discharge efficiency. As will become apparent to those skilled in the art, the present invention addresses the latter approach through a discovery that leads to an increased high-rate discharge efficiency for EMD, which is the electrochemically active material commonly used in the positive electrode of an alkaline electrochemical cell. To better understand the present invention, a description is provided below of the manner by which EMD is commonly produced.
EMD that is suitable for use in an alkaline electrochemical cell generally includes about 92 percent manganese dioxide (MnO2). A large percentage of the remainder of the EMD is Mn2O3. EMD additionally includes many different impurities at relatively low levels. Ideally, the EMD includes as high a percentage of MnO2 as possible, to maximize cell service performance.
MnO2 is a naturally occurring compound that is mined as an ore. The ore generally includes fairly high levels of impurities. The specific impurities and levels of impurities may vary considerably. Nevertheless, a typical analysis of a raw ore shows that it contains the following:
MnO2xe2x80x9475 percent
Fexe2x80x943-4 percent
Kxe2x80x940.7-0.8 percent
Moxe2x80x9415-20 ppm
Coxe2x80x941200 ppm
Nixe2x80x94600 ppm
Al2O3xe2x80x946 percent
SiO2xe2x80x943 percent
The raw ore is then processed through many different purification steps to arrive at a suitable form of EMD. The first step is a calcining process. The MnO2 in the raw ore is insoluble in acid, which makes it difficult to further process the raw ore. Thus, the calcining process is used to convert the insoluble MnO2 to manganese oxide (MnO), which is soluble in sulfuric acid. To produce the MnO (calcined ore), methane is used as a reagent in the presence of significant heat to cause the reduction of MnO2 to MnO as shown in the formula below: 
A typical analysis of a calcined ore is:
MnOxe2x80x9460 percent
MnO2xe2x80x941-2 percent
Fexe2x80x943-4 percent
Kxe2x80x940.7-0.8 percent
Moxe2x80x9415-20 ppm
COxe2x80x941200 ppm
Nixe2x80x94600 ppm
However, the levels of impurities can vary considerably, depending upon the raw ore.
The calcining process is typically carried out in brick-lined rotary kilns operated at about 1000xc2x0 C. The calcined ore is then cooled and transferred to storage bins.
The next step in the process is known as the leaching process. There are several different leaching processes. One of the more common ones is known as the Jarosite process. In the Jarosite leaching process, the stored calcined ore is dissolved in sulfuric acid in order to remove iron (Fe) and potassium (K) impurities. The following reactions may take place in the leaching process: 
The leaching process generally takes place in one or more leach tanks. The initial pH in the leach tank is about 0.9. The calcined ore is added incrementally to slowly raise the pH to 4.2. As the pH rises, the mix undergoes the following reactions: 
The first of the three above reactions is known as the Jarosite reaction. At the end of the leach bath, polymer may be added to the tanks to help settle suspended solids. These solids are then removed by filtering. The clear solution having the solids removed is then processed by the third step known as the sulfiding process.
The sulfiding process is typically performed in a holding tank. The sulfiding process is used to precipitate heavy metal impurities (M), such as molybdenum (Mo), cobalt (Co), and nickel (Ni). The solution that overflows from the filter in the leaching process is mixed with sodium hydrosulfide (NaSH). The NaSH is converted to H2S, which then precipitates the impurities as sulfides. Thus, the solution undergoes the following reactions: 
The solid sulfides are then filtered out through two rotary vacuum drum filters. The filter material is diatomaceous earth. The resultant filtrate constitutes what is known as purified cell feed.
The cell feed is fed into one or more plating cells. Each plating cell may include many negative and positive plating electrodes. Each plating cell includes at least one negative and one positive electrode. Titanium is often used for the negative electrodes, and copper or lead can be used for the positive electrodes. Current flows through each cell to deposit the EMD on the negative electrode. Through this process, MnO2 is plated onto the titanium negative electrode via the following reactions: 
The cell bath is maintained at the desired temperature and acid concentration. The total process is a closed-loop system. The plating cells generate sulfuric acid and plate MnO2 while the leach process consumes the sulfuric acid that is generated during the plating process and dissolves manganese.
After terminating the plating, the EMD is stripped off the negative electrode. The material is then ready for the finishing operation, which may include milling, washing and/or neutralizing. Washing and neutralizing may be done before, during or after milling. For example, in one finishing operation chunks of EMD are crushed to about xc2xe inch (1.9 cm) average external diameter. This material is then sent to one or more neutralization tanks. In these tanks, an alkaline solution such as NaOH or KOH is used to increase the pH of the material to a predetermined level to meet finished product specifications. After the material is neutralized, it is milled and screened to the desired particle size distribution. The EMD is then ready for use in cell manufacture. The EMD may be first mixed with a conductive agent and impact-molded directly into the cylindrical can of the battery or may be mixed with a conductive agent and pre-molded into rings that may subsequently be inserted into the cell.
It is an aspect of the present invention to provide an electrochemical cell, specifically an alkaline electrochemical cell, having improved high-rate discharge properties. To achieve this and other aspects and advantages, the electrochemical cell according to the present invention comprises a negative electrode, an electrolyte, and a positive electrode comprising electrolytic manganese dioxide having a pH-voltage of at least about 0.860 volt. The high-rate discharge may further be improved by using electrolytic manganese dioxide having less than about 250 parts per million (ppm) of potassium impurities by weight.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification and claims.